The two younger men swam to shore to get help but the father was unconscious when he was found during the major search operation.

Officers and a lifeguard performed CPR on the man, but he could not be revived.

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The Westpac Life Saver Rescue Helicopter at a major rescue operation in Kurnell after a boat overturned.Credit:Seven News

"They were fishing at the time and have just been caught out by the waves, they've just become too close to the rocks and have capsized," Inspector Darren Wood said.

"We've been informed that they were not wearing lifejackets at the time.

"It is unfortunate ... it is important that all boaters remember to wear their lifejacket when on a vessel this summer. It is your best chance of survival should you be thrown into the water."

Inspector Wood also encouraged all boaters "to understand the conditions and understand where the reefs are and have some local knowledge of where they are fishing".

One man was winched to the top of the cliff where he was treated for serious lacerations to his fingers.Credit:Seven News

Police said one of the surviving men's thumb was severed. The man had a few other scrapes but would require surgery for his hand. The other man on the rocks was able to walk to safety and had no major injuries.

The pair were taken to hospital in a stable condition and were interviewed by police.

The Westpac Life Saver Rescue Helicopter at a major rescue operation in Kurnell after a boat overturned.Credit:Westpac Life Saver Rescue Helicopter

Westpac Rescue Helicopter chief executive Stephen Leahy said it "doesn't matter what the conditions might be" or how good they are at swimming, people need to wear lifejackets if they were going offshore.

"It's too common: people believe they're reasonably good swimmers, but it doesn't take much for a boat to roll over," he said.

"That's why it's incredibly important: lifejackets are an essential item that needs to be worn."

A report will be prepared for the coroner on the man's death.

Police were calling for any witnesses to contact Crime Stoppers on 1800 333 000.